Ducted fan gas turbine engines conventionally comprise a core engine which drives a comparatively large diameter fan positioned at the upstream end of the core engine. The fan is made up of a plurality of aerofoil blades which can be vulnerable to damage as a result of foreign object ingestion by the engine. In most cases, the fan is sufficiently robust to withstand the effects of such foreign object ingestion without suffering major damage and is able to continue operating, although, perhaps at reduced efficiency.
On very rare occasions, the fan could be damaged to such an extent that parts of one or more of its aerofoil blades may be lost. This usually necessitates the shutting-down of the engine involved to minimise damage to the engine and to the aircraft carrying it. However, the imbalance in the fan created by the blade loss generates extremely high loads which must be at least partially absorbed before the engine is allowed to run-down to windmilling speed. Windmilling speed is the speed at which the engine rotates in a non-operative condition as a result of its motion through the atmosphere.
One way in which fan imbalance load absorption can be achieved is by the use of so-called "fuse bolts". Typically the main bearing supporting the upstream end of the shaft carrying the fan is supported radially from the remainder of the engine structure via a plurality of axially extending fuse bolts. In the event of major fan imbalance, the resultant high radial loads cause the fuse bolts to fracture in shear and allow the fan and its shaft to orbit about the engine's longitudinal axis. This continues as the engine is allowed to run down to windmilling speed. Such arrangements are disclosed in GB2079402 and GB2130340.
Unfortunately, under certain circumstances, the vibration resulting from fan imbalance that still exists at windmilling speed can still be extremely severe. This is due mainly to the natural frequency of the fan and the lack of radial stiffness of the fan assembly.